Radio-frequency (RF) transceivers are found in many one-way and two-way communication devices, such as portable-communication devices, (cellular telephones), personal-digital assistants (PDAs) and other communication devices. A RF transceiver transmits and receives signals using whatever communication methodology is dictated by the particular communication system within which it is operating. For example, communication methodologies typically include amplitude modulation, frequency modulation, phase modulation, or a combination of these. In a typical global-system for mobile communications (GSM) mobile-communication system using narrowband time-division multiple access (TDMA), a Gaussian minimum shift keying (GMSK) modulation scheme is used to communicate data.
The deployment of new wireless systems presents unique challenges to mobile handset designers. In order to reap the full benefit of expanded capacity and increased data bandwidth, it is desirable for the next generation of handsets to work using multiple communication systems.
WCDMA (Wideband Code Division Multiple Access) is a radio access scheme used for third generation (3G) cellular systems that are being deployed world wide. The 3G systems support high-speed Internet access, video and high-quality image transmission services. In WCDMA systems, the CDMA air interface is combined with GSM based networks including enhanced data rates for GSM evolution (EDGE) networks. The EDGE standard is an extension of the GSM standard.
Conventional WCDMA and GSM/EDGE receiver architectures use a pair of circuits driven by mixers to separate components of the receive signal. Generally, sine and cosine components of the received carrier signal are applied to mixers to extract the separate components. This “mixing” of the carrier signal produces what is referred to as in-phase or “I” signal component and a quadrature phase or “Q” signal component. These I and Q signal components are filtered, gain/phase adjusted, and finally sent to a baseband digital signal processor to extract the communicated data.
In cellular communications systems, a signal transmitted from a base station is generally constant and at a level that provides for a region of overlap with its nearest-neighbor base stations in the cellular network. Consequently, a mobile transceiver that is relatively close to a base station receives a receive channel signal with a higher signal strength than a mobile transceiver that is positioned further from the base station. Accordingly, the receiver, for such a mobile transceiver, needs a large dynamic range to ensure that the mobile transceiver can process the full range of power levels across receive signals without creating distortion. This is typically accomplished using some manner of receive signal gain adjustment.
Prior art approaches to adjust gain include automatic gain control (AGC) systems implemented in the baseband portion of a transceiver. These prior art baseband approaches do not account for the intermittent presence of an interfering signal or blocker in the RF portion of the transceiver. For example, in digital-video broadcasting-handheld systems (DVB-H), a desired receive signal may be suddenly adversely affected or “jammed” with a GSM transmitter blocker that compresses the circuitry in the front end of the receiver. Furthermore, these prior art systems must constantly monitor and correct for changing signal conditions due to relative movement between the mobile transceiver and the nearest base station and relative movement of other objects in the path between the mobile transceiver and the nearest base station. These digital gain control systems often fail to provide accurate power control in environments where signal strength is rapidly varying over a large dynamic range.